Correlation between low/high affinity ratios for 5-HT 1A receptors and intrinsic activity

Abstract

G protein-coupled receptors exist in G protein-coupled and -uncoupled forms that exhibit high and low affinity for agonists, respectively. Consequently, affinity differences of a compound for the high vs. the low affinity state of a receptor have been used to estimate its intrinsic activity at that receptor. We examined the affinity of a series of compounds for 5-hydroxytryptamine 1A (5-HT 1A) receptor sites labeled with 0.2 nM [ 3H](±)-8-hydroxy-2-(di- n-propylamino)tetralin ([ 3H]8-OH-DPAT) (high affinity), or with 0.25 nM [ 3H]4-(2′-methoxy-)-phenyl-1-[2′-( N-2″-pyridyl)- p-fluorobenzamido]ethyl-piperazine ([ 3H] p-MPPF) in the presence of 100 μM guanylylimidodiphosphate (Gpp(NH)p) (low affinity) in rat hippocampal membranes. For a variety of 5-HT 1A receptor ligands, the low/high affinity ratio (ranging from 110 for 5-HT to 0.12 for spiperone) was in good agreement with their reported intrinsic activity. Positive rank correlations were found between low/high affinity ratios and intrinsic activities ( E max values) reported in the literature. The high efficacy 5-HT 1A receptor agonists, 1[2-(4-fluorobenzoylamino)ethyl]-4-(7-methoxynaphtyl)piperazine (S-14506) and dihydroergotamine, however, had similar, high affinity for both G protein-coupled and -uncoupled forms of the receptor. The Hill coefficients for both compounds were markedly higher than 1.0, suggesting that positive cooperativity could be responsible for the unexpected results. The 5-HT 1A receptor agonist activity of dihydroergotamine and S-14506, assessed by measuring the inhibition of forskolin-stimulated cAMP accumulation, was blocked completely by pertussis toxin, reinforcing the suggested involvement of an inhibitory G protein in their effects. Taken together, the results suggest that, although the low/high affinity ratio of a ligand for 5-HT 1A receptors generally covaries with its intrinsic activity, dihydroergotamine and S-14506 may interact with 5-HT 1A receptors in a manner different from that of other 5-HT 1A receptor agonists. Their effects, however, appear to be G i protein-dependent.